FIG. 1 illustrates the construction of a conventional copper vapor laser apparatus as described on "MANUFACTURE OF COPPER VAPOR LASER", pages 60 to 66 of "Laser Research" published in March 1981. In this figure, the apparatus is constituted by a discharge tube 100, a pair of opposite electrodes 1, 1 for generating gas discharge, an inner tube 2 having therein copper particles 3 from which copper vapor is generated, an outer sleeve 4, a heat-insulating layer 5 for preventing the heat generated by discharge between the pair of electrodes 1, 1 from being lost, windows 6 disposed on the sides of the pair of electrodes 1, 1 to take out a laser beam, and an insulating break 7 disposed in an intermediate portion of the outer sleeve 4. A pulse circuit 200 is constituted by a charging capacitor 8 connected to the outer sleeve 4 of the discharge tube 100 through a connecting line a, a charging reactor 9 connected in series to the capacity 8, a diode 10 having an anode connected to one end of a high voltage power source 11 and a cathode connected to the charging reactor 9, a thyratron 12 connected to a connecting portion between the charging reactor 9 and the charging capacitor 8, and a charging resistor 14 connected in parallel to the discharge tube 100. The other end of the high voltage power source 11 is connected to the outer sleeve 4 through a connecting line b. Further, a pulse control circuit 13 is connected to a grid of the thyratron 12.
In the conventional metallic vapor laser apparatus constructed as above, a high voltage is charged from the high voltage power source 11 to the charging capacitor 8 through the diode 10, charging reactor 9 and charging resistor 14. Next, when the thyratron 12 is turned on by the pulse control circuit 13, the high electric voltage charged by the charging capacitor 8 is applied to the pair of opposite electrodes 1, 1 through the outer sleeve 4, so that gas discharge is generated within the inner tube 2. The heat energy generated by the discharge within the inner tube 2 is held by the heat-insulating layer 5 so that the temperature of the inner tube 2 is increased to a high temperature of about 1500.degree. C., thereby vaporizing the copper particles 3 and filling the inner tube 2 with the copper vapor. Electrons in the discharge plasma are accelerated by the gas discharge formed by the pair of opposite electrodes 1, 1, and collide with the copper atoms filled within the inner tube 2, and excite the energy level of copper atoms to an upper energy level of a first resonance level, thereby forming an inverted population since the number of atoms excited to a low energy level of a metastable level is less. The copper atoms at the upper level drop to the low energy level while causing laser oscillation and further are gradually released to the ground level from the low level. The above-mentioned operations are repeatedly performed at several kHz. The laser beam is taken out of the discharge tube through the window 6. The insulating break 7 insulates the high electric voltage mentioned above.
The copper atoms at the low level are gradually released to the ground level by the collision between a wall of the inner tube 2 and the excited atoms when the diameter of the inner tube 2 is small, and by super-elastic collision between the atoms at the low level and the electrons at a low speed when the diameter of the inner tube 2 is large. The relaxation time from the low level to the ground level is several hundred .mu. seconds which is a long time.
In the conventional copper vapor laser apparatus mentioned above, since the relaxation time from the low level to the ground level is several hundred .mu. seconds which is a very long time, if a next pulse is applied to the discharge tube after completion of the relaxation, the number of pulse repetitions becomes small, and if the number of pulse repetitions are large, the inverted population becomes incomplete and the efficiency in copper vapor laser is reduced since the number of atoms at the low level is large at the time of pulse application.
Further, in the conventional pulse laser apparatus constructed as above, when the reactor 9 is reduced to increase repetition frequency to several kHz or the discharge load is varied, the voltage of the charging capacitor 8 exceeds a holding voltage of the thyratron 12 before the thyratron 12 is recovered in operation so that the thyratron 12 cannot be turned off and a short-circuit current flows from the high voltage direct current power source 11 to the thyratron 12, thereby interrupting the high voltage direct current power source 11 in operation and greatly reducing the reliability of the pulse laser.
To solve the problems mentioned above, an object of the present invention is to provide a metallic vapor laser apparatus having greatly improved reliability and efficiency even when the number of pulse repetitions is large, by promoting the relaxation from the low energy level to the ground energy level between respective pulses to make the inverted population at the subsequent pulse more complete.